1. Field of the Invention
The present invention relates to an alloy catalyst for purifying exhaust gas and, particularly, to an alloy catalyst having excellent nitrogen oxide purification characteristics in the lean atmosphere, which has improved catalytic activity because first catalytic metal particles are coated with a second metal coagulated phase, and to a process for producing the same.
2. Description of the Prior Art
Precious metals such as Pt, Pd and Rh have heretofore been used either alone or in combination as a catalyst component for catalysts for purifying exhaust gases of automobiles, etc. The precious metals are generally supported on a catalytic carrier. The problem of removing nitrogen oxide from an oxygen-rich atmosphere in a diesel engine exhaust gas has recently become important. However, conventional catalysts for purifying an exhaust gas have a limitation on their purifying capacity. Therefore an alloy catalyst is considered to be suitable as a catalyst for removing nitrogen oxide (No.sub.x) from an oxygen-rich atmosphere. Japanese Unexamined Patent Publication (Kokai) No. 60-184922, for example, discloses, as a known technology in this field, a method wherein an oxide layer is formed as a catalyst for purifying an exhaust gas of diesel engine, and the oxide layer is immersed in a solution so that at least two kinds of catalysts are simultaneously supported. Moreover, Japanese Unexamined Patent Publication (Kokai) No. 4-284824 discloses a method for purifying nitrogen oxide by supporting Pt and a second component Sr, for an oxygen-excessive exhaust gas, on .gamma.-Al.sub.2 O.sub.3. Furthermore, Japanese Unexamined Patent Publication (Kokai) No. 9-103679 discloses a purification catalyst for diesel engines which is used at upstream and downstream positions of a path for the exhaust gas and which comprises fine particles upstream and coarse particles downstream.
However, conventional alloy catalysts as catalysts for purifying No.sub.x in an oxygen-excessive atmosphere have problems such as those described below.
(1) Since the conventional alloy catalysts are solid solution type, the combination of elements is restricted.
(2) When at least two precious metals are supported on a catalytic carrier by adsorption, coprecipitation, or the like procedure, an alloy cannot be formed from those elements which form a solid solution with difficulty.
Accordingly, the development of a technology by which an alloy catalyst as a catalyst for purifying nitrogen oxide in an oxygen-rich atmosphere can be produced relatively simply, and which improves the catalytic activity, is required.